The ternary polar intermetallic phase Mg5.231(8)Sm0.769(8)Sb4 has been obtained from solid-state reactions at 700-850 degrees C in sealed Ta or Nb containers when the synthetic conditions took into account its characteristic incongruent melting point. The compound crystallizes in the trigonal space group P (3) over bar (Z = 1) with a = 4.618(1) angstrom and c = 14.902(6) angstrom in a structure that derives from that of Mg3Sb2 (anti-La2O3 type). This composition appears to be near the lower limit of Sm content, and solutions with appreciably higher Sm contents are also stable [Mg6-xSmxSb4, x <= 1]. The result provides the first example of a superstructure of a Mg3Sb2-like structure with a doubled c axis induced by ordering a mixture of the larger divalent Sm and Mg ions separately within alternate layers of octahedral sites. Still larger proportions of Sm also give rise to a second solid solution region in the parent Mg3Sb2 type structure (P (3) over bar m1), Mg3-ySmySb4, 0 <= y <= 1. Retention of the same 3e(-) valence electron counts per antimony in all of these phases suggests that the compounds remain electron precise and Zintl phases. Analogous compounds with Ca, Sr, or Ba substitution have evidently not been investigated.